Tens of thousands of prisoners could sue if they are not allowed to vote in this year's general election, the government is being warned.

The UK, one of the few countries in the world to impose a blanket ban on prisoners voting, must give those held behind bars the vote in order to comply with a European legal ruling, dating back six years. However, ministers have been procrastinating on the issue, fearing that it will prove unpopular with the electorate.

But with the election now looming, a high-profile coalition of judicial experts, politicians and lawyers will tomorrow warn the government that continuing the ban could result in widespread legal challenges after the election.

The Barred From Voting campaign – organised by the Prison Reform Trust and Unlock, the national association of reformed offenders, and supported by the Prison Governors Association and the civil rights group, Liberty – claims that barring the UK's 83,000-strong prison population from voting "has no place in a modern democracy and is legally and morally unsustainable".

"What message does the government think that the continued defiance of the rule of law sends to prisoners as well as the other countries in the Council of Europe?" said Lord Ramsbotham, the former chief inspector of prisons and a supporter of the campaign.

Last year the government was forced to acknowledge that some prisoners must be allowed to vote, although it is likely some categories of offender will continue to be barred.

The decision followed a lengthy legal battle dating back to 2005 when a UK prisoner, John Hirst, brought a case before the European Court of Human Rights that ruled the government was in violation of Article 3, Protocol 1 of the European Convention on Human Rights, which guarantees the right to vote. A subsequent government appeal was rejected, prompting the Ministry of Justice to launch a series of consultation exercises on the issue that were condemned by opponents as simple delaying tactics.

Liberal Democrat peer and human rights expert Lord Lester has warned the government its delaying tactics "will lead to costs to the taxpayer if prisoners take their cases to Strasbourg for this gross violation of a binding judgment".

The Liberal Democrat leader, Nick Clegg, senior Conservative politicians including former home secretary Lord Hurd, and senior Labour figures such as David Winnick MP and Lord Corbett of Castle Vale, have criticised the blanket ban on votes for prisoners.